This invention relates generally to a discharge device providing a spectral light source, and more particularly to a hollow-cathode type light source.
Hollow-cathode light sources are used in atomic absorption spectroscopy. Such light sources provide high intensity, sharply defined spectral lines for many analytical applications. One conventional hollow-cathode type light source design is shown in U.S. Pat. No. 3,264,511. In these devices the spectral light is produced as a result of a concentrated discharge which takes place between an anode and a hollowed or bore portion of a cathode. The bore part of the cathode contains the atomic species which generates the desired spectral light output. The discharge should be confined to the bore part of the cathode to achieve most efficient and stable operation.
The noted patent and the following listed U.S. patents owned by the assignee of this invention show various arrangements that include means to prevent high voltage arcing and to promote light output and stability: U.S. Pat. Nos. 4,071,802; 3,855,491; 3,725,716; and 3,563,655. These patents all include mica discs extending transversely within the outer glass envelope of the device including at least one disc between the anode ring and the cathode, as well as insulating sleeves on the anode lead-ends.
Additionally, known commercial forms of hollow-cathode devices have an interior glass sleeve structure which loosely encases the cylindrical cathode circumferentially and necks down and tightly encases the cathode lead-in, to the end of reducing spurious discharges between anode structure and cathode.
Nevertheless, even with the internal glass sleeve arrangement, intermittent spurious discharges occur between the sides and base of the cathode and the inside face of the glass sleeve presented thereto. The front face of the cathode around the hollow or bore is also exposed to unwanted discharges. Because the desired spectral light output is primarily derived from inside the hollow or bore, the spurious discharges within the glass sleeve will cause variations in light output, and the front face discharges can, in themselves, reduce maximum attainable light output by as much as 50%.
Accordingly, it is the aim of the invention to reduce light output noise and fluctuations by shielding all external cathode surfaces where unwanted discharges can occur, and to thereby concentrate all available discharge current to the inside of the bore so that light output is increased.